Recent advances in biology and medicine have led to the development of laser beam microsurgery on cells. The laser beam is well adapted to micro-manipulation of small objects, such as single cells or organelles. It provides the advantage of non-contact ablation, volatilization, sterilization and denaturing, cutting, and other forms of thermal and actinic-light treatment. The four parameters of focal spot size, laser wavelength, pulse duration, and laser power, provide a variety of regimes suitable for different applications. One example use of laser beam microsurgery is the application of laser beams to the treatment of the mammalian oocyte and embryo. However, laser beam microsurgery in a number of inverted or upright microscopes can be utilized for many different surgical or medical applications.
In further detail of the example application, the early-stage mammalian embryo is contained within a protective layer, the zona pellucida (“ZP”). The ZP is relatively analogous to the shell of a hen's egg. This proteinaceous ZP layer is of varying thickness, typically 10 to 20 μm, and of varying hardness. The embryo remains within the ZP during development from the single-cell to the blastocyst stage, at which point the embryo breaks out of the ZP and implants itself into the uterine wall.
It has been found that certain embryos, typically those from older mothers or embryos that have been frozen for storage, frequently have much tougher ZP layers than younger-origin or untreated embryos. Consequently, when the time comes for the embryo to emerge from the ZP, there may be a significant impediment in the tougher layers, which have to be traversed. If the embryo fails to hatch in the limited time available, it will be lost and fertility will fail.
Assisted hatching derives from the observation that fertility can be augmented by generating holes or gaps in the ZP through which the embryo can more easily emerge. This has been done using mechanical or ultrasonic cutting, chemical erosion (acidified Tyrodes solution), or by laser ablation of part of the ZP.
A laser can be used to produce a trench in the edge of the ZP layer, penetrating through (or almost through) the ZP thickness. The trench creates a weakened region through which the embryo will later emerge. Lasers of many types can ablate the ZP. A laser of wavelength λ=1480 nm, which is strongly absorbed in water, has been found to be effective for thermolysing the ZP. The laser can be used in pulses relatively short enough to avoid significant thermal conduction into the nearby embryo blastomeres, and at the same time avoid a chemical effect on the cellular chemistry, since it is in the non-actinic infrared wavelength region. The ZP is removed out to a radius around the laser beam determined by the local temperature history during the laser pulse.
The same laser system can be used to ablate a larger region of ZP so that an intact blastomere can be removed for external analysis. The laser is used, in this case, in a series of pulses directed at adjacent parts of the ZP to erode away a larger region. Typically, a gap is opened until a pipette can be introduced to suck out a blastomere. The embryo is relatively resilient, and generally survives both hatching and biopsy ZP ablation.
Another related application of the laser system is in direct removal of the polar body for genetic analysis. Removal can be done either at the oocyte stage (first polar body), or after fertilization at the embryo stage (second polar body). Both polar bodies can be used to derive information on the genetic composition of the embryo. The procedure is analogous to laser assisted biopsy, except that in this case only the ZP layer and not the perivitelline membrane is penetrated, since the polar body remains between the perivitelline layer and the ZP. Additional related applications include transfer of part or all of the nucleus (nuclear transfer or transgenetic engineering), and ablation and destruction of part or all of the cell nucleus or oocyte spindle (e.g. in a cell to be used as the recipient of nuclear transfer). All of these applications benefit from the precise ablation capabilities of the laser system.
The above applications are by way of example and should not be construed as limiting the possible uses of the invention. The present invention can be applied in any field where a laser beam is used with a microscope assembly.